1. Technical Field
The present invention relates to a charging control method for a secondary battery and a battery charger, and more particularly to technology for charging a secondary battery such that a desired state of charge (SOC) can be obtained.
2. Background Art
Secondary batteries such as nickel-metal hydride batteries and lithium ion batteries have been installed in motor-driven vehicles including hybrid electric vehicles, electric vehicles, and so on, as a power source. Although the secondary batteries are shipped from factories in a fully-charged state or a desired state of charge (e.g. SOC=60%), the capacities of the batteries are reduced due to self discharge when the batteries are left in storages and so on for a relatively long time period. In such a case, it is necessary to perform periodic auxiliary charge, and highly accurate auxiliary charge which enables a desired state of charge (e.g. SOC=60%) is desirable.
JP 2001-351698 A concerns technology regarding a lead-acid battery, and discloses measuring an open-circuit voltage OCV at the time of discharge of a storage battery to calculate the state of charge SOC based on a relationship between the OCV and the SOC which has been preset, and charging the storage battery based on the calculated SOC such that a desired SOC can be achieved.
JP 8-17477 A discloses estimating the capacity of a secondary battery by measuring a battery voltage obtained when the secondary battery is charged with a predetermined charging current, based on a relationship in which, when the charging current is the same, the smaller the capacity of a battery, the more the battery voltage increases. This document also discloses estimating the capacity of a secondary battery by measuring a battery voltage obtained when the secondary battery is discharged by a predetermined discharging current, based on a relationship in which, when the discharging current is the same, the smaller the capacity of a battery, the more the battery voltage decreases.
However, in the case of calculating the SOC based on the relationship between the OCV and the SOC and performing auxiliary charge based on the calculated SOC, the following problem arises. Specifically, as this technology is based on the precondition that SOC is calculated in a highly accurate manner based on the OCV, it is not possible to perform auxiliary charge to a desired SOC when the calculation accuracy of the SOC is lowered. In particular, when the SOC which is calculated based on the OCV is lower than the actual SOC, auxiliary charge is continued based on this low SOC, resulting in overcharge of the secondary battery. Although there exists a certain relationship between the OCV and the SOC in the initial state (immediately after manufacturing), it is likely that such a relationship is significantly affected by the history of usage of the secondary battery, and particularly by the temperature of the battery during storage when the battery is stored for a long period of time.
FIG. 6 illustrates, with marks ●, a relationship between an OCV value (an OCV value before start of charge, [V]) and an SOC value (a capacity value before charge, [Ah]), when a plurality of secondary batteries are stored under various storage conditions. Each secondary battery is charged with a constant current (e.g. 1 A) by an appropriate amount under a predetermined environment, and is then discharged to a predetermined discharge termination voltage per cell (e.g. 1.0V for a nickel-metal hydride battery). Based on the discharge capacity and the charge capacity associated with this charging and discharging, the SOC value (a capacity value before charge) is calculated. The various storage conditions specifically include various storage temperatures of 0° C., 25° C., and 45° C., respectively. By plotting pairs of OCV values and SOC values and applying first-order approximation by using a least-square method, the following can be obtained:y=7.2028x−54.21wherein x is an OCV value (OCV value before start of charge, [V]) and y is an SOC value (capacity value before charge, [Ah]), and a correlation coefficient is R2=0.6432.
Further, FIG. 6 also illustrates actual SOC values (%), by plotting the values with marks ▪, in a case in which auxiliary charge is performed from the SOC value (capacity value before charge, Ah) which is calculated from this approximation line until a desired SOC value (SOC in the initial state=60%, which corresponds to 3.9 Ah) is obtained. As illustrated in FIG. 6, even when auxiliary charge is performed based on the SOC value (capacity value before charge, Ah) obtained by calculation from the approximation line, the actual SOC value (%) does not necessarily match SOC=60%, and varies between 49.9% and 72.2%.
As described above, even when the SOC (capacity value before charge, Ah) is calculated based on the relationship between the OCV (an OCV value before start of charge) and the SOC (capacity value before charge, Ah) and auxiliary charge is performed based on the calculated SOC, it is difficult to perform highly accurate auxiliary charge with respect to all the secondary batteries stored in various temperature conditions.
On the other hand, FIG. 7 illustrates, with marks ●, a relationship between an OCV value (an OCV value before start of charge, [V]) and the actual SOC (%) when auxiliary charge is performed until the terminal voltage of 8.43V (a terminal voltage value corresponding to SOC=60% in the initial state) is reached, in a case in which a plurality of secondary batteries are stored under various storage conditions. Each secondary battery is charged with a constant current (e.g. 1 A) by an appropriate amount under a predetermined environment, and is then discharged to a predetermined discharge termination voltage per cell (e.g. 1.0V for a nickel-metal hydride battery). Based on the discharge capacity and the charge capacity associated with this charging and discharging, the SOC value (a capacity value before charge, Ah) is calculated. An amount of charging electricity to the voltage of 8.43V is added to the SOC value thus calculated to calculate the SOC value (%) when reaching 8.43V. FIG. 7 also illustrates, with marks ♦, a relationship between an OCV value (an OCV value before start of charge, [V]) and the amount of charging electricity when auxiliary charge is performed until the terminal voltage of 8.43V is reached. As illustrated in FIG. 7, even when auxiliary charge is performed until the terminal voltage reaches a predetermined value (8.43V in this case) based on a precondition that there is a certain relationship between the terminal voltage and SOC, the actual SOC value does not necessarily match SOC=60%, and varies between 49.6% and 72.8%. One of the causes of such a variation can be considered to be various relationships between the terminal voltage and the SOC depending on different storage conditions of the secondary battery.
FIG. 8 illustrates relationships between the SOC and the terminal voltage when auxiliary charge is performed after the secondary battery is stored under various storage temperature conditions. In FIG. 8, the battery capacity value corresponds to SOC (Ah), and 3.9 Ah corresponds to SOC=60%. Here, the storage temperatures and the charging conditions at the time of auxiliary charge are as follows:
(1) Store at 25° C. and auxiliary charge at 25° C.
(2) Store at 25° C. and auxiliary charge at 30° C.
(3) Store at 25° C. and auxiliary charge at 20° C.
(4) Store at 25° C. and auxiliary charge at 35° C.
(5) Store at 25° C. and auxiliary charge at 15° C.
(6) Store at 45° C. and auxiliary charge at 25° C.
(7) Store at 0° C. and auxiliary charge at 25° C.
As can be understood from FIG. 8, when the storage temperatures vary, the terminal voltage corresponding to SOC=60% also varies and is not determined uniquely. As such, because the relationship between the terminal voltage and the SOC can vary, it is difficult to obtain desired SOC even when auxiliary charge is performed based on the terminal voltage.